Before jumping head first (there is no other way) into how entanglement is used in Quantum computer and how it is about to change the world, there are a lot of premise we might want to touch on. So tag along and we’ll try and explain. Orite.

What is Quantum Computing ?

The secret to a quantum computer’s power lies in its ability to generate and manipulate quantum bits, or qubits. The ability to prepare sizeable multi-qubit entangled states with full qubit control is a critical milestone for physical platforms upon which quantum computers are built.

Quantum computing uses the laws of quantum mechanics to process information. A traditional computer uses long strings of “bits,” which encode either a zero or a one. A quantum computer, on the other hand, uses quantum bits, or qubits. The difference being that a qubit is a quantum system that encodes the zero and the one into two distinguishable quantum states. But, because qubits behave quantum, we can exploit on the phenomena of “superposition” and “entanglement.”

Superposition simply means that a quantum system can be at various states at the same time while entanglement means that there is a relation between quantum particles ie., electorns, protons, etc even if they are far distances apart.

Here is my fav YouTube channel for these sort of things, Kurzegesagt, that will explain it in about 7 minutes.

How entanglement is used in quantum computer actually?

It is harnessed by creating a far larger state space than possible with classical bits. The 2^n possible states of n classical bits constitute the vertices of an n-dimensional cube, e.g. the 8 states 000 through 111 when n = 3. Those vertices constitute the classical n-dimensional information space or state space for n bits.

If qubits weren’t entangled they’d work independently like classical bits, except that each qubit encodes not one Boolean truth value but instead two complex numbers up to a constant factor. Since complex numbers are comprised of two reals, that would be four real dimensions but when scale is factored out (by a complex number) you get in effect a spherical 2-dimensional space like the surface of the Earth with only compass directions, no notion of up or down. This is called the Bloch sphere. Whereas classical bits have two values, the Bloch sphere has two dimensions. A key difference from classical mechanics is that in quantum mechanics every classical value gets its own dimension.

Entanglement and its Applications

Quantum entanglement is regarded as one of the key hallmarks separating quantum from classical systems. Its significance is fundamental, being the subject of the ‘spooky’ correlations noted by Einstein, Podolsky and Rosen (EPR) and used by Bell to rule out non-local hidden variable descriptions of quantum mechanics.

More recently the utility of entanglement has become apparent, with quantum entanglement viewed as a useful resource to aid information processing tasks. The simplest form of entanglement, EPR pairs, enable tasks such as quantum cryptography, super-dense coding, teleportation and entanglement swapping. More complex, multi-qubit examples of entanglement enable one-way quantum computation, entanglement assisted error correction, and as a tomographic resource.

What do current researches say on how entanglement is used in quantum computer?

According to this paper, Quantum entanglement often is seen as a key ingredient if quantum computers are to demonstrate an advantage over classical computers. In particular, if a quantum system is not highly entangled it can often be simulated efficiently on a classical computer.

Multi-qubit entanglement is therefore a fundamental property for potential quantum computers to demonstrate, if they are to ultimately outperform classical computation. To this end large multi-qubit entangled states have been demonstrated in a number of experimental systems. On systems with full qubit control, entanglement has been shown on up to a 16-qubit superconducting system and a 20-qubit ion trap system, while genuine multipartite entanglement has been shown on up to an 18-qubit photonic system, 12-photon system and 12-qubit superconducting system.

Over the past few years, a series of quantum devices have been released by IBM that comprise five to twenty superconducting qubits and can be accessed via their cloud service. Of particular interest here is the device IBM Q Poughkeepsie, which exhibits improved error rates over previous devices.

In addition, companies such as Microsoft (MSFT), Alibaba (BABA), Tencent (TCEHY), Nokia (NOK), Airbus, HP (HPQ), AT&T (T) Toshiba, Mitsubishi, SK Telecom, Raytheon, Lockheed Martin, Righetti, Biogen, Volkswagen and Amgen are researching and working on applications of quantum computing.

The Chinese quantum computing system, called Jiuzhang, completed in 200 seconds what they estimate one of the world’s most powerful supercomputers would have taken 2 billion years to solve. The supercomputer in question was China’s Sunway TaihuLight, the third fastest in the world. This puts Jiuzhang’s quantum advantage at 10¹⁴. The results were published on December, 2020.

Here is a video by Veritasium on Quantum Entanglement

Terms that are thrown around when this subject comes up

  1. Quantum Supremacy: It’s the point at which a quantum computer can complete a mathematical calculation that is demonstrably beyond the reach of even the most powerful supercomputer.
  2. Quantum Decoherence: The interaction of qubits with their environment in ways that cause their quantum behavior to decay and ultimately disappear is called decoherence.
  3. Paradox: It is a measurement made on either of the particles apparently collapses the state of the entire entangled system—and does so instantaneously, before any information about the measurement result could have been communicated to the other particle and hence assured the “proper” outcome of the measurement of the other part of the entangled pair.
  4. Bell’s Theorem : It proves that quantum physics is incompatible with local hidden-variable theories.
  5. Pure states, Ensemble : Bunch of maths. Please read here.

An Infographic to explain it

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Infographic Source: here


As if augmented reality, blockchain etc weren’t complex enough. But in laymanish terms, two subatomic particles in quantum world are connected like siamese twins ie, literally entangled across some distance and that property can be exploited in computing as explained above and that’s how entanglement is used in quantum computer.



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